A Cuckoo In The Nest
by Joss Teagan
Summary: A creature has moved into Baker Street, treating Sherlock and John as extensions of its own body. The torture and beatings isn't the worst they'll have to endure- there's worse coming. Complete one-shot. Mentions of Johnlock. Not for the faint-hearted. Mentions of non-con.


**Disclaimer: I have been reliably informed that I have no legal claim on Sherlock BBC. Reivew if you like this fic! It's a complete one-shot, unbeat'd but I have looked over it.**

John wasn't sure when the entity had first arrived at Baker Street. He reckoned it was the unseasonably cold night they'd had in July, when the wind had howled, branches slamming on the window panes, making the glass tremble, vibrating. Sherlock felt sure it was earlier, he thought the entity had been eyeing them for a while, biding its time, looking for the right opportunity to strike. It didn't matter. All that mattered was that it was here now. And it wasn't planning on leaving anytime soon.

He reminisced as he ran a wet bar of soap along his arms and legs, hearing Sherlock's restless pacing outside the bathroom door. He wouldn't enter the room, not when there was running water. The entity didn't like water, it was one of the signs Sherlock had noted. He'd had difficulty conveying it but they'd worked it out in the end. It was a relief to be able to come in here and know he was reasonably safe, for as long as the taps ran and the water flowed. He was untouchable. He liked to remember Sherlock before the entity, when Sherlock was Sherlock, when he could laugh without malice and act kindly. Huh. The Yarders would laugh at that, Sherlock being kind. But he was, in his way. Allowing John to drag him to the pub, letting Mr Hudson fuss over him and listening to her endless anecdotes. Mrs Hudson didn't come up anymore. Something had changed, the entity left a strange and constant coldness in the rooms, no matter how high they turned up the thermostat or whether the radiator was on or not. John supposed it was good that she didn't visit anymore. At least she was safer than they were.

Thud. Thud. Thud. John looked up, his heart hammering a tattoo in time with the knocking on the door. It sounded like Sherlock was throwing himself at the door. John knew the entity wouldn't enter the room with the water running, but it was warning him. _If you don't open the door, I'll make Sherlock break the door down. And if I have to break his body to do it…so be it._ With a resigned sigh, John dried himself off, got dressed and prepared to face the body of his friend. At least Sherlock wouldn't be in pain for a while, the entity didn't like to hurt the body it was sharing.

The moment the door creaked open, Sherlock burst in, all bouncing curls and snarls, grabbing John roughly and dragging him out of the refuge of the bathroom. John concentrated on not looking into the Not-Sherlock's eyes, because he knew they'd be completely black, a wet, slick sable that resembled wet paint, looking as if any minute, shiny black would run from his eyeballs and down his face. Not-Sherlock was licking him, running a panting tongue over his neck and John felt dread because It always did that just before it was about to change bodies. Grooming the body It'd take. It would take turns, occupying one body and then the next, swapping every few days. John would endure hell as the face of his best friend sneered at him, a soft growl on the lips and blood on the teeth. And then It would enter him, through the mouth, and Sherlock would gasp, gaining consciousness as Not-John broke him. The memory loss of days possessed was a godsend, but the fear and dread in Sherlock's eyes was not. The entity occasionally occupied other things, the air would crackle with electricity, every available surface giving Sherlock and John static shocks when touched, and the light bulbs would flicker constantly, on and off, on and off. Like fairy lights. But It preferred bodies.

John's mouth was forced open by Not-Sherlock and his lips covered his. There was a hot burst on his tongue, the sensation of something huge being forced down his throat, so big his eyes watered and he gulped and spluttered, then…darkness.

Sherlock gasped, slumping against the wall as the colours of John's eyes ran, deep brown darkening to black. He stepped back, backing away slowly as if John was an angry bull. But already, John was heading downstairs to the fridge. Sherlock sighed. The demon preferred to possess him, not John, probably because of Sherlock's ability to go days without sleeping and eating. _When It inhabited John, Sherlock would find It tearing into raw hunks of steak, blood dripping down Its chin as It wolfed down the meat. It drank water, occasionally blood. Sherlock shivered, remembering one time It had inhabited John, and clamped John's lips on Sherlock's neck, teeth digging in until the skin broke, lapping at blood as it oozed sluggishly out. It never tried to kill them. Sherlock wasn't sure if this was good or not. He also remembered the first time he woke up, after the first time he'd been possessed. Ridiculous. He still had trouble believing it. But John had looked so frightened, and recoiled from him in disgust when he'd rested a hand on John's arm. It was at that time that Sherlock wished he wasn't so clever- his deductions forced him to see the things he didn't want to. The bloodstains on John's trousers, the terror in his eyes. He'd raped him and he couldn't even remember it. He had tried to help John up, fix him as the creature flitted from the television to the light fixtures. He'd fixed John's limp, why couldn't he fix this?_

_Then the acceptance came, managing this as best as they could. They'd looked for the answer in books, Sherlock with more enthusiasm than John, whispering as the shadow danced over their heads. How to get this out, get this evil thing out. Holy water and spells promised the answer. There just wasn't time, even speaking to each other was getting harder. Sherlock knew he could find a way to cure them of this curse, if given time. He knew that running away wasn't an option, leaving John to face that. Once they'd run, both of them, John's sweating palm clasped in his but they'd barely got across the street before the lights had flickered. Mrs Hudson's lights. They'd had to go back. The demon had possessed John, made him beat Sherlock up, not the punches he'd received when they were trying to get into Irene Adler's property, this was hard and brutal. Sherlock had heard each snap of each bone as it broke, before he felt each fiery tendril of white-hot pain wrap around each limb. Then the demon had made John put his own hand in the fire. Then it had pounced into the television, the black screen now replaced with static. Punishment. John and Sherlock had understood._

_Sometimes it rewarded them, usually when they kept the lights on all hours of the day and when John would pick up lamb, pork and steak from the butcher's shop. Sometimes he'd even unwrap it, ready. When it rewarded them, Sherlock would find the possessed body of his friend being gentler. John would stare at him with unblinking wet black eyes, as he pushed him to the ground, giving every inch of his body raspy licks, almost an affectionate gesture. Sherlock had never experienced quite what John had, the torturous rape that lasted for hours, but there was rough sex, easier if he put up with it. He was never harmed as badly as John ever was, because the demon preferred possessing him, but he still remembered the foreign feeling of pain down below, slick blood pouring out of him, staining the carpet. He'd had to look away as the demon licked it up. Other times, the demon fucked him hard, purring like a cat as it raked nails, John's nails, up his body. Once, Sherlock woke and he was inside John, hard and aroused, deep inside, naked, sweaty. The demon had temporarily left them, let them be themselves, just for a bit. A present, then. That was the thing about knowing you could die at any minute, it made you appreciate what you had. Sherlock had mouthed "I love you" as he took John in quick but shallow thrusts, not daring to test the demon's patience._

Sherlock joined the demon in the kitchen. John was eating ravenously, swallowing chunks of meat, empty packets and tins surrounded him, his black eyes flicking left and right, restlessly. Something had changed. Sherlock didn't know what. As he watched John, he noticed something. One, John had gotten fatter, although it was mainly along the middle. Damn, how long had It been inhabiting Sherlock, he had no clue. He used to be able to keep track of the days, but time was lost with possession, and he'd find days and weeks lost of his life. Maybe he'd been possessed longer than he thought. His quest for knowledge made him brave and he interrupted the feasting. "Creature. The thing that's using John. What's happening?"

The creature's head snapped up and it raised a finger to its lips. "Soon," it whispered. Then it curled up on the floor to sleep. Sherlock knew he was going to get no answers so he ran to his bedroom to research demonology some more. He didn't think the demon had ever slept before, although he couldn't be sure.

Hours later, screams brought him from his mind palace. He ran downstairs to find John, still on the floor, and yes, thank goodness, it was John, John with his blond hair streaked liberally with grey, his honest, trusting brown eyes and his face lined with pain…?

Sherlock leapt over empty packets of food, grasping John's hand. He was burning up. "John, John, look at me!"

"Hurts," John croaked, his hands clutching his backside. Sherlock threw him a worried glance, reaching forward to yank John's trousers down. John tried to stop him but crumpled over, face pale, as another wave of pain ripped through him.

"John, hold on, I can help, I'm here-"

"HURTS!"

"I know, but I'm here, John, I'm always here, please, let me-" Sherlock swallowed, seeing a fleshy lump emerging from John's behind. "John, please don't be scared, but, I can't be sure, but I think- your insides are coming out," Tears were falling freely from his face, splashing onto John's oatmeal sweater, but this couldn't happen, John couldn't die, he was the only thing keeping Sherlock going, the one person who knew what he was going through-

"I have to, have to, ugnh…" John's face creased in pain and he threw his head back, with a ragged whimper.

"Just hold still, I'll call an ambulance," Sherlock didn't care if the demon, wherever it was right now, would try to stop them. He would fight to the death to get care for his friend.

"Can't, I have to- push…"

"WHAT? No!" Sherlock said wildly, looking around the room for his mobile phone, for a towel or blanket to mop up the blood. He was also keeping out an eye for the creature but it was being strangely inconspicuous. "Don't push!"

"Have to…uh…" John gave an almighty bellow even as Sherlock shouted at him to lie still. John tensed and then his body suddenly sagged. Still. So still. An icy chill tingled on Sherlock's skin and he crawled over on hands and knees, to where John lay.

"John…" he whispered, hoping and hoping he'd see John breathe. Not knowing _what_ he'd see. Sherlock knelt beside John, watching his lax face. John's eyelids flickered and he coughed. "Oh, John!" Sherlock said in relief, grabbing John's hand. He was alive! He was fine.

Something, just a sound, one sound, made by neither Sherlock nor John. Sherlock knew this because his own mouth was drawn in an ecstatic smile at seeing his friend alive. And John's own mouth was closed. A gurgle. A choky, wet sound. Sherlock's smile faded and he looked down, to the blood-stained carpet. The thing he'd taken to be John's innards now lay on the floor, but it was moving. A red, wet, bloody ball, no, not a ball, a _thing_, not human but humanoid, with arms and legs and thin, leathery wings that gently fluttered as it kicked its legs. A baby. A demon baby. Sherlock's awestruck face turned upwards as the light bulb flickered, back and forth. The tiny creature giggled. A baby. A cuckoo in the nest.

John caught sight of it and was transfixed with horror, his mouth moving silently. Sherlock could see the spawn reflected in John's eyes, and when John asked Sherlock in a whisper, from the side of his mouth "Sherlock, what do we do?" , it was Sherlock's turn to be silent. He had no answers.

**Hope you enjoyed that, or as much as you can enjoy something like that! Reviews are love!**


End file.
